Method for forming electrode on diamond for electronic devices

ABSTRACT

Disclosed is a method of forming electrodes on diamond comprising the steps of: forming a mask pattern on diamond or diamond film; performing a treatment of the diamond surface by a plasma of inert gases; forming an electrode film on the whole surface of the specimen; and removing the mask, thereby forming a specified pattern of the electrodes. By this method, it is possible to form electrodes having high adhesion to diamond and diamond film for electronic devices.

This application is a continuation of application Ser. No. 08/219,422,filed on Mar. 29, 1994, (abandoned), which is a continuation ofapplication Ser. No. 07/958,754, filed Oct. 9, 1992 (Abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for forming electrodes withhigh adhesion to diamond for diamond electronic devices such as heatsinks, diodes, and transistors.

2. Description of the Related Art

Diamond has high hardness and stability against heat, radiation, andchemicals. It also has a large band gap of 5.4 eV. Diamond iselectrically insulating, but becomes semiconducting by doping. Thus,diamond is expected to be used for electronic devices which can beoperated at high temperatures. Furthermore, diamond is transparent overa wide wavelength range between ultraviolet and infrared, and is usedfor optical windows.

Diamond films formed by vapor deposition methods also have theabove-mentioned excellent characteristics, and are used for coatings oncutting tools and speaker diaphragms, as well as for electronicapplications such as heat sinks, diodes, and transistors.

It is well known that diamond film can be formed by vapor phasesynthesis using plasma chemical vapor deposition (CVD) at low cost.Similarly for bulk diamonds, synthesized diamond film becomessemiconducting by doping with an impurity such as boron (B).

In the fabrication of electronic devices using diamond, it is necessaryto form electrodes on the surface of diamond. Known methods of formingelectrodes on the surface of diamond are as follows:

(I) Ohmic contacts were obtained by a method of vapor-deposition ofmetals liable to form carbides, e.g. Ta, on diamond followed by a heattreatment by electron beam irradiation, thus forming a carbideintermediate layer at the interface between diamond and the metalelectrode, thereby enhancing the adhesion A. T. Collons et al. Formationof electrical contacts on insulating and semiconducting diamonds,Diamond Research, 1970!. This method will be referred to hereafter asMethod I.

(II) Ohmic contacts were obtained by a method of vapor-deposition ofmetals liable to form a carbide, e.g. Mo, on diamond followed by a heattreatment using a vacuum furnace K. L. Moazed et al. Material ResearchSociety Symposium Proceedings 162, P.347, 1990).

Using this method, electrodes with high adhesion were formed byheat-treating metal films having multiple layer structures such asAu/Pt/Ti, Au/Ti, or the like. This method will be referred to hereafteras Method II.

(III) Metal electrodes were formed after irradiating Ar ions on thesurface of diamond, thereby transforming the surface thereof intographite C. B. Child, Fourth Annual SD 10/IST-ONR Diamond TechnologyInitiative Symposium, 19891!. By this method, Au electrodes with highadhesion were formed on the surface of diamond. This method will bereferred to hereafter as Method III.

However, the above-described methods have the following problems:

In Method I, it is necessary to perform the heat treatment using anelectron beam generator at high vacuum, which is expensive and timeconsuming. Thus, this method is disadvantageous in terms of productioncost.

In Method II, the metal electrodes which have been vapor-deposited ondiamond must be heated at high temperature (e.g. 800° C.) to form acarbide. This causes stress at the interface between the electrode andthe diamond due to the difference in thermal expansion coefficients.Thus, this method has the disadvantage that the electrodes are thermallydamaged and tend to peel off from the diamond.

Also, in a heat treatment at high temperatures, the surface of a diamondis transformed into graphite. Thus, in this method, low resistanceparasitic channels are liable to be formed on the surface of a diamond.Furthermore, similarly for Method I, since this method employs a vacuumfurnace in the heat treatment, this process is very time consuming.

In Method III, an expensive ion beam generator has to be operated athigh vacuum, which is costly and time consuming. Furthermore, when aprocessed sample is subjected to ultrasonic cleaning, the electrodespartially or totally peel off because of poor adhesion.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a method for formingelectrodes with high adhesion to diamond for electronic devicefabrication.

To achieve the objective, in a preferred mode of the present invention,there is provided a method for forming electrodes on electronic devicesusing diamond comprising the steps of: first performing a plasmatreatment on the surface areas of a diamond where electrodes are to beformed by a plasma of inert gases, and then depositing electrodematerials on the surface areas of the diamond.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objectives, features, and advantages of the presentinvention will become more apparent from the following description takenin connection with the accompanying drawings, in which:

FIG. 1a to 1d are cross-sectional views showing a method for formingelectrodes on a diamond film formed on a substrate, according to thefirst example of the present invention (Note that when bulk diamonds areused, substrate 1 in FIGS. 1a-1d is unnecessary.); and

FIG. 2 is a typical view showing a magnetron sputtering apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior to the description of the preferred embodiments, the function ofthe present invention will be explained.

The present applicants have found that a high adhesion of electrodes todiamond can be obtained by performing a plasma treatment of a diamondsurface where the electrodes are to be formed, by a plasma of inertgases. By this treatment, hydrocarbon, water, and the like adhering on adiamond surface are removed by exposing the surface to the plasma, withthe adhesion of the electrodes to the diamond surface becoming veryhigh.

This invention can be employed not only for bulk diamond crystals, butalso for diamond films synthesized by CVD.

Also, by this treatment, a very thin surface layer of a diamond surfacearea is transformed into graphite by the damage due to plasma exposure.This contributes to the improvement of the adhesion of the electrodes tothe diamond surface, because the adhesion of metal films to graphite isgenerally better than that of metal films to diamond.

The above-mentioned inert gases include rare gases such as He, Ne, Ar,Kr, or Xe, or chemically inert gases such as N₂. During the plasmaprocessing, a bias voltage, either direct or alternating current (DC orAC), may be applied on the sample holder to enhance the cleaning effect.

When a metal liable to form a carbide is used for an electrode on adiamond surface, which is then followed by a heat treatment undersuitable conditions, a carbide layer is formed by the reaction betweenthe metal electrodes and diamond, thus enhancing the adhesion of theelectrode to diamond. This also creates electronic defect levels at theinterface, which contribute to the ohmic nature of the contact.

Furthermore, diffusion of electrode materials occurs by a heattreatment, which also results in defect levels at the interface betweenthe electrodes and diamond. Consequently, the ohmic electrodes areliable to be formed.

The temperature and duration of the heat treatment are dependent on thekinds of electrode material: for the case of Au/Ti, a heat treatment canbe done at 400° C. for one hour.

Finally, and technologically more importantly, the process inventedhere, namely, the plasma treatment of a diamond surface followed byelectrode formation, can be done in the same vacuum chamber forsputtering, for instance, as explained later in Examples 1-3. Therefore,there is no loss of time in the device fabrication process nor need forextra costly apparatus. Moreover, the present invention can be employednot only for bare diamond surfaces, but also for diamond surfacesphotolithographically patterned by such mask materials as photoresist,metals, or ceramic films without damaging the mask materials during theplasma treatment under proper conditions.

The present invention will be more apparent by way of the followingexamples with reference to the accompanying drawings.

EXAMPLE 1

FIG. 1a to 1d are cross-sectional views showing a method of formingelectrodes according to this example of the present invention in theorder of fabrication processes when the electrodes are formed on adiamond film 2. However, note that no substrate is necessary whenelectrodes are formed on bulk diamond crystals.

FIG. 2 is a view of a standard magnetron sputtering apparatus used forthe plasma treatment of the diamond surface. In FIG. 2, a sample 11(consisting of a diamond film and a photoresist film) was mounted on asample stage 12 in a vacuum chamber 10. The electrode 13 was grounded,and high frequency voltage was applied from an RF power supply 14 to thesample stage 12 to generate a plasma. The sample 11 was thussurface-treated by the plasma. In the above, the chamber 10 wasevacuated to approximately 10⁻⁶ Torr, and then an inert gas (or amixture of inert gases) was fed at the flow rate of 10 cc/min at a gaspressure of 2 mTorr into the chamber 10. The frequency of a power supply14 was 13.56 MHz, the RF output was 100 W, and the plasma treatment timewas 1 min.

In this example, the experiments were done in the following procedure:

(1) First, as shown in FIG. 1, a semiconducting diamond film 2 wasformed on a substrate 1 to a thickness of approximately 2 μm bymicrowave plasma CVD. As a reaction gas for diamond film deposition, CH₄was diluted by H₂ (concentration: 0.5%), and a doping gas of B₂ H₆(concentration: 0.01ppm) was added. During the CVD, the substratetemperature was maintained at 800° C., and the gas pressure in thereaction chamber was 35 Torr. The CVD was continued for 7 hours. Thediamond film 2 thus obtained was then coated with a photoresist film 3using the standard photolithography technique.

(2) As shown in FIG. 1b, openings according to the specified electrodepattern were formed on the photoresist film 3 using a photolithographytechnique. The specimen was then placed in the above-mentionedsputtering apparatus (FIG. 2), and plasma-treated using the conditionsdescribed above. As a result, only the surface area of the diamond film2 were treated by a plasma through the openings.

(3) The metal electrodes were formed using the same magnetron sputteringapparatus as shown in FIG. 1c. In this case, a target of electrodematerial was placed on the electrode 13, and a DC bias voltage wasapplied on the target side. The substrate side was grounded so that anelectrode film 4a of a bilayer structure, made of a Ti film and an Aufilm, was formed on the whole surface of the substrate. The formationconditions of the Ti film and the Au film are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                       Ti film   Au film                                              ______________________________________                                        DC discharge condition                                                                         0.8 A, 390 V                                                                              0.2 A, 535 V                                     film deposition time                                                                           15 sec.     1 min.                                           film thickness   400 Å   2000 Å                                       ______________________________________                                    

(4) As shown in FIG. 1d, the photoresist film 3 was removed togetherwith the electrode film 4a on the photoresist film 3. Thus, a pattern ofAu/Ti electrodes 4 was obtained.

The sample thus obtained was subjected to ultrasonic cleaning using purewater for 8 min., but the electrodes did not peel off. As a comparativeexample, a sample obtained by forming electrodes without the surfacetreatment by plasma was subjected to ultrasonic cleaning in the sameconditions as in this example. However, more than 90% of the electrodepattern peeled off after approximately 10 sec.

EXAMPLE 2

A similar experiment as in Example 1 was carried out. Here, an S_(i) O₂film was deposited on a diamond film by magnetron sputtering in place ofthe photoresist film of Example 1, and the openings were formed on theSiO₂ film according to a predetermined pattern by etching using aphotolithography technique. Au/Ti electrodes were then deposited on thesurface of the sample. The sample thus obtained was subjected to theultrasonic cleaning in the same condition as in Example 1. However, noelectrodes peeled off.

In the surface treatment using the inert gas plasma in Examples 1 and 2,the RF output applied to the electrode 11 was changed. When the RFoutput was 600 W, the electrode pattern formed by the photoresist filmwas severely damaged, which made it impossible to form the electrodes bythe lift-off method. Therefore, when a high RF output was required, anS_(i) O₂ film was preferably used in place of the photoresist film as amask material.

EXAMPLE 3

The experiment was repeated in a similar manner as in Examples 1 and 2.In this case, the heat treatment was done after the plasma treatment andthe electrode formation, under the condition of a temperature of 400°C., a pressure of 10⁻⁶ Torr, and a processing time of 30 min. The samplewas then subjected to an ultrasonic cleaning in the same conditions asdescribed in Example 1. However, no separation of the electrodes wasfound. In this example, the contact resistance between each metalelectrode and the diamond film was reduced by approximately 2 orders ofmagnitude as compared with the cases without heat treatment.

What is claimed is:
 1. A method for forming an electronic device,comprising the steps of:coating a surface of a diamond with aphotoresist; followed by photolithographically forming openings in thephotoresist according to a specified electrode pattern, thereby exposinga region or regions of said surface through said openings; followed bytreating said region or regions of said surface with a plasma, withoutdamaging said photoresist; followed by depositing a thin metal film onsaid photoresist and said region or regions of said surface; followed byremoving the photoresist from said surface; followed by performing aheat treatment on said diamond with said thin metal film thereon;wherein said plasma is generated by an alternating current, said plasmaconsists essentially of at least one gas selected from the groupconsisting of helium, neon, argon, krypton, xenon and nitrogen, saidthin metal film comprises titanium or gold, and said plasma is generatedby a RF output of less than 600 W.